1. Field of the Invention
The present invention relates to a linear drive apparatus that is used in a machine tool or a measuring apparatus having a guide, a slider that is moved along the guide by a driving mechanism, and a magnetic measurement unit for measuring the relative position of the slider relative to the guide.
2. Related Art
In a linear drive apparatus comprising a guide and a slider that is moved along the guide by a driving mechanism, an optical measurement unit is generally used to measure the relative position of the slider relative to the guide. Such an optical measurement unit generally has a light source, an optical scale, and a light receiving section. However, there are problems with such optical measuring units in that they are inappropriate for measurement, especially for in-process measurement, in a degraded environment where coolant or chips are present, and that they cannot be applied to wet processing of small articles.
On the other hand, it is known that a magnetic measurement unit having a magnetic scale and a detector are less sensitive to a degraded environment than an optical measurement unit. A magnetic measurement unit has a magnetic scale and a detector that are opposed to each other and change position relative to each other, and the detector detects a signal from the magnetic scale in accordance with the position of a slider for measuring the position of the slider.
However, when the magnetic scale is mounted to a slider or the like that is driven by a linear motor mechanism, the magnetic scale is likely to be affected by the magnetic force generated by a coil or a magnet, and this may lead to a problem of detecting an accurate position. Therefore, a magnetic measurement unit including a magnetic scale has to be mounted at a position sufficiently far away from a coil or magnet, so that precision higher than a certain level cannot be achieved in detecting a position.
In order to overcome the above-described problem that a magnetic measurement unit is likely to be affected by the magnetic force in the surroundings, and therefore cannot detect an accurate position, a method in which a magnetic shield is mounted to the magnetic scale, and a method as disclosed in Japanese Patent Publication No. 11-215795 (JP-A-11-215795) in which another magnet is provided between the magnetic scale and the coil or the magnet so as to cancel the magnetic force generated by the coil or the magnet, have been proposed. These methods, however, lead to another problem that, since a magnetic shield or a coil is added to the apparatus, it becomes difficult to reduce the size or weight of the apparatus.